The present invention relates to a thin film capacitor and a method of forming the same, and more particularly to a thin film capacitor having an improved bottom electrode to be used in a semiconductor integrated circuit.
A high temperature oxygen atmosphere is necessary to form a high dielectric constant thin film such as SrTiO.sub.3 or (Ba, Sr) TiO.sub.3 which shows several hundreds dielectric constant at room temperature. Such the high dielectric constant thin film is deposited on a bottom electrode of a stable electrically conductive oxide such as RuO.sub.2.
In 1994, International Electron Devices Meeting Technical Digest, pp. 831-834, it is disclosed that a polycrystalline RuO.sub.2 thin film is deposited by a reactive sputtering method on a TiN thin film serving as a diffusion barrier layer for subsequent patterning the laminations of the TiN thin film and the polycrystalline RuO.sub.2 thin film to define an RuO.sub.2 TiN storage electrode before an SrTiO.sub.3 thin dielectric thin film is deposited on the RuO.sub.2 /TiN storage electrode by a chemical vapor deposition method.
In 1995, International Electron devices Meeting Technical Digest pp. 119-122, it is disclosed that an Ru thin film is deposited on a TiN diffusion barrier layer before a polycrystalline RuO.sub.2 thin film is deposited by a reactive sputtering method on the Ru thin film to form a RuO.sub.2 /Ru/TiN storage electrode.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional thin film capacitor having a bottom electrode of an electrically conductive oxide. A TiN diffusion barrier layer 11 is provided on a substrate 9. A polycrystalline conductive oxide bottom electrode 24 is provided on the TiN diffusion barrier layer 11. A dielectric film 25 is provided on the polycrystalline conductive oxide bottom electrode 24. A top electrode 26 is provided on the dielectric film 25.
The polycrystalline conductive oxide bottom electrode 24 has a pillar-shaped or particle-shaped crystal structure with a size of several tens nanometers, for which reason a surface of the polycrystalline conductive oxide bottom electrode 24 also has an roughness of the same size as the crystal structure. Particularly, this problem with the surface roughness of the bottom electrode is significant when the polycrystalline conductive layer serving as the diffusion barrier layer is provided on the substrate and the polycrystalline conductive oxide layer serving as the bottom electrode is provided on the diffusion barrier layer. A field concentration may appear at convex portions of the surface of the bottom electrode. The dielectric film is unlikely to be deposited within concave portions of the surface of the bottom electrode, whereby voids are likely to be formed at the concave portions of the surface of the bottom electrode. For those reasons, a leakage of current may be caused and a breakdown voltage may be reduced.
In Japanese laid-open patent publication No. 2-46756, it is disclosed that a polycrystalline silicon film is subjected to an ion-implantation of As in order to amorphize a surface region of the polycrystalline silicon film before a dielectric film is formed on the amorphized surface region of the polycrystalline silicon film. FIG. 2 is a fragmentary cross sectional elevation view illustrative of a conventional thin film capacitor having a bottom electrode having an amorphized surface region. A silicon oxide film 102 is provided on a silicon substrate 101. A bottom electrode 103 is formed on a selected region of the silicon oxide film 102, wherein the bottom electrode 103 comprises an undoped polycrystalline silicon film and a surface region which comprises an amorphous layer 108. A silicon nitride film 104 is provided which extends on the amorphous layer 108 and unselected regions of the silicon oxide film 102. A silicon oxide film 105 is provided on the silicon nitride film 104. A polycrystalline silicon top electrode 106 is provided on the silicon oxide film 105.
FIGS. 3A through 3E are fragmentary cross sectional elevation views illustrative of a conventional method of forming the above conventional thin film capacitor having the bottom electrode having the amorphized surface region.
With reference to FIG. 3A, a silicon oxide film 102 is formed on a silicon substrate 101. An undoped polycrystalline silicon film is deposited by a chemical vapor deposition method on the silicon oxide film 102 for subsequent patterning the undoped polycrystalline silicon film.
With reference to FIG. 3B, the patterned undoped polycrystalline silicon film is then subjected to an ion-implantation of an impurity such as As to amorphize a surface region of the patterned undoped polycrystalline silicon film, thereby forming an amorphous silicon layer 108. As a result, a bottom electrode 103 comprising the undoped polycrystalline silicon film and the amorphous silicon layer 108 is formed on a selected region of the silicon oxide film 102.
With reference to FIG. 3C, a silicon nitride film 104 is formed which extends on the amorphous layer 108 and unselected regions of the silicon oxide film 102.
With reference to FIG. 3D, a silicon oxide film 105 is formed on the silicon nitride film 104.
With reference to FIG. 3E, a polycrystalline silicon top electrode 106 is formed on the silicon oxide film 105.
The dielectric of the capacitor includes the silicon oxide film which has a small dielectric constant. This silicon oxide film reduces an effective capacitance, whereby it is difficult to obtain desired characteristics of the film capacitor. Further, the ion-implantation of As for forming the amorphous layer 108 is labored and time-consuming process.
In the above circumstances, it had been required to develop a novel thin film capacitor free from the above problems and a novel method of forming the same.